Lubricants in commercial use today are prepared from a variety of natural and synthetic base stocks admixed with various additive packages and solvents depending upon their intended application. The base stocks can include, e.g., Groups I, II and III mineral oils, gas-to-liquid base oils (GTL), Group IV polyalpha-olefins (PAO) including but not limited to PAOs made by using metallocene catalysts (mPAOs), Group V alkylated aromatics (AA) which include but are not limited to alkylated naphthalenes (ANs), silicone oils, phosphate esters, diesters, polyol esters, and the like.
Manufacturers and users of lubricating compositions desire to improve performance by extending oil drain life of the lubricating composition. Extended drain life is a highly desirable marketing feature of lubricating compositions, especially Group IV/Group V lubricating compositions.
Degree of oxidation of the lubricating composition, also referred to as oxidation stability, affects the oil drain life of the lubricating composition. Oxidative degradation of lubricating composition can lead to damage of metal machinery in which the lubricating composition is used. Such degradation may result in deposits on metal surfaces, the presence of sludge, or a viscosity increase in the lubricating composition.
The kinematic viscosity of a lubricating composition is directly related to the antioxidation performance and degree of oxidation of the lubricating composition. A lubricating composition being used in machinery has experienced oxidative degradation when the kinematic viscosity of lubricating composition reaches a certain level, and the lubricating composition needs to be replaced at that level. Improving the oxidation stability and antioxidation performance of the lubricating composition improves the oil drain life by increasing the amount of time the lubricating composition can be used before being replaced. Various approaches are used to improve the antioxidation performance and extend the oil drain life of Group IV/Group V lubricating compositions. The approaches typically involve increasing the antioxidant additive concentrations of the lubricating composition.
U.S. Pat. No. 6,180,575 to Nipe and assigned to Mobil Oil Corporation discloses lubricating compositions comprising antioxidant additives and API Group II-V base stocks, such as a PAO base stock and alkylated naphthalene base stocks. The antioxidant additives include phenolic antioxidants, such as ashless phenolic compounds, and neutral, or basic metal salts of phenolic compounds. Typical of the dialkyl dithiophosphate salts which may be used are the zinc dialkyl dithiophosphates, especially the zinc dioctyl and zinc dibenzyl dithiophosphates (ZDDP). These salts are often used as anti-wear agents but they have also been shown to possess antioxidant functionality. The antioxidant additives of the '575 patent also include amine type antioxidants, alkyl aromatic sulfides, phosphorus compounds such as phosphites and phosphonic acid esters, and sulfur-phosphorus compounds such as dithiophosphates and other types such as dialkyl dithiocarbamates, e.g. methylene bis(di-n-butyl)dithiocarbamate. The antioxidant additives may be used individually or in combination with one another.
Nonetheless, there is still a need for lubricant base stock blends having improved oxidation stability.